A caldera is a large cauldron-like depression that forms following the
evacuation of a magma chamber/reservoir. When large volumes of magma
are erupted over a short time, structural support for the crust above
the magma chamber is lost. The ground surface then collapses downward
into the partially emptied magma chamber, leaving a massive depression
at the surface (from one to dozens of kilometers in diameter).
Although sometimes described as a crater, the feature is actually a
type of sinkhole, as it is formed through subsidence and collapse
rather than an explosion or impact. Only seven known caldera-forming
collapses have occurred since the start of the 20th century, most
recently at
BárðarbungaBárðarbunga volcano in Iceland.[1]

Etymology[edit]
The word comes from Spanish caldera, and
LatinLatin caldaria, meaning
"cooking pot". In some texts the English term cauldron is also used.
The term caldera was introduced into the geological vocabulary by the
German geologist
Leopold von BuchLeopold von Buch when he published his memoirs of his
1815 visit to the Canary Islands,[note 1] where he first saw the Las
Cañadas caldera on Tenerife, with Montaña
TeideTeide dominating the
landscape, and then the
Caldera de TaburienteCaldera de Taburiente on La Palma.
CalderaCaldera formation[edit]

LandsatLandsat image of Lake Toba, on the island of Sumatra,
IndonesiaIndonesia (100
km/62 mi long and 30 km/19 mi wide, one of the world's largest
calderas). A resurgent dome formed the island of Samosir.

A collapse is triggered by the emptying of the magma chamber beneath
the volcano, sometimes as the result of a large explosive volcanic
eruption (see Tambora in 1815), but also during effusive eruptions on
the flanks of a volcano (see
Piton de la FournaisePiton de la Fournaise in 2007) or in a
connected fissure system (see
BárðarbungaBárðarbunga in 2014–2015). If enough
magma is ejected, the emptied chamber is unable to support the weight
of the volcanic edifice above it. A roughly circular fracture, the
"ring fault", develops around the edge of the chamber. Ring fractures
serve as feeders for fault intrusions which are also known as ring
dykes. Secondary volcanic vents may form above the ring fracture. As
the magma chamber empties, the center of the volcano within the ring
fracture begins to collapse. The collapse may occur as the result of a
single cataclysmic eruption, or it may occur in stages as the result
of a series of eruptions. The total area that collapses may be
hundreds or thousands of square kilometers.
Mineralization[edit]
Some calderas are known to host rich ore deposits. One of the world's
best-preserved mineralized calderas is the
Sturgeon Lake Caldera in
northwestern Ontario, Canada, which formed during the Neoarchean
era[2] about 2,700 million years ago.[3]
Explosive caldera eruptions[edit]
Further information: Explosive eruption
If the magma is rich in silica, the caldera is often filled in with
ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma
has a high viscosity, and therefore does not flow easily like basalt.
As a result, gases tend to become trapped at high pressure within the
magma. When the magma approaches the surface of the Earth, the rapid
off-loading of overlying material causes the trapped gases to
decompress rapidly, thus triggering explosive destruction of the magma
and spreading volcanic ash over wide areas. Further lava flows may be
erupted.
If volcanic activity continues, the center of the caldera may be
uplifted in the form of a resurgent dome such as is seen at Cerro
Galán, Lake Toba, Yellowstone, etc., by subsequent intrusion of
magma. A silicic or rhyolitic caldera may erupt hundreds or even
thousands of cubic kilometers of material in a single event. Even
small caldera-forming eruptions, such as
KrakatoaKrakatoa in 1883 or Mount
Pinatubo in 1991, may result in significant local destruction and a
noticeable drop in temperature around the world. Large calderas may
have even greater effects.
When
Yellowstone CalderaYellowstone Caldera last erupted some 650,000 years ago, it
released about 1,000 km3 of material (as measured in dense rock
equivalent (DRE)), covering a substantial part of
North AmericaNorth America in up
to two metres of debris. By comparison, when
Mount St. HelensMount St. Helens erupted
in 1980, it released ~1.2 km3 (DRE) of ejecta. The ecological
effects of the eruption of a large caldera can be seen in the record
of the
Lake TobaLake Toba eruption in Indonesia.
Toba[edit]
Main article: Lake Toba
About 74,000 years ago, this Indonesian volcano released about
2,800 km3 DRE of ejecta, the largest known eruption within the
QuaternaryQuaternary Period (last 1.8 million years) and the largest known
explosive eruption within the last 25 million years. In the late
1990s, anthropologist Stanley Ambrose[4] proposed that a volcanic
winter induced by this eruption reduced the human population to about
2,000–20,000 individuals, resulting in a population bottleneck (see
Toba catastrophe theory). More recently several geneticists, including
Lynn Jorde and
Henry Harpending have proposed that the human species
was reduced to approximately five to ten thousand people.[5] However,
there is no direct evidence that the theory is correct, and there is
no evidence for any other animal decline or extinction, even in
environmentally sensitive species.[6] There is evidence that human
habitation continued in
IndiaIndia after the eruption.[7] The theory in its
strongest form may be incorrect.
Eruptions forming even larger calderas are known, especially La Garita
CalderaCaldera in the
San Juan MountainsSan Juan Mountains of Colorado, where the 5,000-km3
Fish Canyon
TuffTuff was blasted out in a single major eruption about
27.8 million years ago.
At some points in geological time, rhyolitic calderas have appeared in
distinct clusters. The remnants of such clusters may be found in
places such as the
San Juan MountainsSan Juan Mountains of
ColoradoColorado (formed during the
Oligocene, Miocene, and
PliocenePliocene periods) or the Saint Francois
Mountain Range of
MissouriMissouri (erupted during the Proterozoic).

SollipulliSollipulli Caldera, located in central
ChileChile near the border with
Argentina, filled with ice. The volcano sits in the southern Andes
Mountains within Chile's Parque Nacional Villarica.[8]

Some volcanoes, such as shield volcanoes
KīlaueaKīlauea and Mauna Loa
(respectively the most active and second largest on Earth, are both on
the island of Hawaii), form calderas in a different fashion. The magma
feeding these volcanoes is basalt which is silica poor. As a result,
the magma is much less viscous than the magma of a rhyolitic volcano,
and the magma chamber is drained by large lava flows rather than by
explosive events. The resulting calderas are also known as subsidence
calderas, and can form more gradually than explosive calderas. For
instance, the caldera atop
Fernandina IslandFernandina Island underwent a collapse in
1968, when parts of the caldera floor dropped 350 meters.[9] Kilauea
CalderaCaldera has an inner crater known as Halema‘uma‘u, which has often
been filled by a lava lake.
During the April 2007 eruption of the
Piton de la FournaisePiton de la Fournaise on the
island of Réunion, the floor of the main crater suddenly dropped
about 300 m. This was attributed to the withdrawal of magma which was
being erupted through a vent lower down on the southern flank of the
volcano.
Another process that may allow a caldera to form can occur if molten
lava can escape through a breach on the caldera's rim.
Extraterrestrial calderas[edit]
Since the early 1960s, it has been known that volcanism has occurred
on other planets and moons in the Solar System. Through the use of
manned and unmanned spacecraft, volcanism has been discovered on
Venus, Mars, the Moon, and Io, a satellite of Jupiter. None of these
worlds have plate tectonics, which contributes approximately 60% of
the Earth's volcanic activity (the other 40% is attributed to hotspot
volcanism).[10]
CalderaCaldera structure is similar on all of these planetary
bodies, though the size varies considerably. The average caldera
diameter on
VenusVenus is 68 km. The average caldera diameter on Io is
close to 40 km, and the mode is 6 km;
Tvashtar PateraeTvashtar Paterae is
likely the largest caldera with a diameter of 290 km. The average
caldera diameter on
MarsMars is 48 km, smaller than Venus. Calderas
on Earth are the smallest of all planetary bodies and vary from 1.6 to
80 km as a maximum.[11]
The Moon[edit]
The
MoonMoon has an outer shell of low-density crystalline rock that is a
few hundred kilometers thick, which formed due to a rapid creation.
The craters of the moon have been well preserved through time and were
once thought to have been the result of extreme volcanic activity, but
actually were formed by meteorites, nearly all of which took place in
the first few hundred million years after the
MoonMoon formed. Around
500 million years afterward, the Moon's mantle was able to be
extensively melted due to the decay of radioactive elements. Massive
basaltic eruptions took place generally at the base of large impact
craters. Also, eruptions may have taken place due to a magma reservoir
at the base of the crust. This forms a dome, possibly the same
morphology of a shield volcano where calderas universally are known to
form.[10] Although caldera-like structures are rare on the Moon, they
are not completely absent. The Compton-Belkovich Volcanic Complex on
the far side of the
MoonMoon is thought to be a caldera, possibly an
ash-flow caldera.[12]
Mars[edit]
Further information: Volcanology of Mars
The volcanic activity of
MarsMars is concentrated in two major provinces:
TharsisTharsis and Elysium. Each province contains a series of giant shield
volcanoes that are similar to what we see on Earth and likely are the
result of mantle hot spots. The surfaces are dominated by lava flows,
and all have one or more collapse calderas.[10]
MarsMars has the largest
volcano in the Solar System, Olympus Mons, which is more than three
times the height of Mount Everest, with a diameter of 520 km (323
miles). The summit of the mountain has six nested calderas.[13]
Venus[edit]
Further information: Volcanology of Venus
Because there is no plate tectonics on Venus, heat is mainly lost by
conduction through the lithosphere. This causes enormous lava flows,
accounting for 80% of Venus' surface area. Many of the mountains are
large shield volcanoes that range in size from 150–400 km in
diameter and 2–4 km high. More than 80 of these large shield
volcanoes have summit calderas averaging 60 km across.[10]
Io[edit]
Further information: Volcanology of Io
Io, unusually, is heated by solid flexing due to the tidal influence
of
JupiterJupiter and Io's orbital resonance with neighboring large moons
Europa and Ganymede, which keeps its orbit slightly eccentric. Unlike
any of the planets mentioned, Io is continuously volcanically active.
For example, the NASA
Voyager 1Voyager 1 and
Voyager 2Voyager 2 spacecraft detected nine
erupting volcanoes while passing Io in 1979. Io has many calderas with
diameters tens of kilometers across.[10]
List of volcanic calderas[edit]
See also: Category:Calderas